There is a present need for an irradiation device that can provide a uniform wide area beam. This would have a number of applications, including the processing of materials requiring electron beam exposure, such as in semiconductor manufacturing, sterilization, curing of polymers, etc. For example, in curing spin-on glass coatings on semiconductor wafers or CVD coatings, an electron beam may be used to drive off organic elements in the coating.
One technical challenge is to generate a uniform plasma that could provide an even areawise amount of energy to irradiate an object. Uniform large area electron beams are usually generated by scanning a small beam across the large area. Frequently, beam energy falls with the square of the radius from a scan center. Alternatively, space charge emission may be used to generate the uniform large area electron beam. This method relies on the voltage and the separation of the cathode and anode elements for the generation of the electron beam without dependence on the thermionic emitter. Beam non-uniformities are common.
In plasma devices, a gas is ionized and the ions bombard a target cathode. In such devices, space charge emission is not possible and the electron density is dependent on the ion density and surface state of the cathode. In a uniform electrical field, the ion extraction from the plasma can be uniform. Yet there is an edge effect where the beam is less dense at the edge of a beam pattern than at the center.
Prior art devices are described in U.S. Pat. Nos. 3,970,892 and 4,755,722. These patents disclose ion plasma electron guns using a vacuum chamber into which a low pressure gas is introduced. A high voltage cathode generates a plasma that is accelerated through control and shield grids into a second chamber containing a high voltage cold cathode. The positive ions bombard the cathode, causing the cathode to emit secondary electrons, forming a beam. The electron beam leaves the gun through a foil window. Control of this beam is accomplished by application of a control voltage between the grid and the grounded housing, to regulate the density of ions bombarding the cathode.
Another electron source is described in U.S. Pat. No. 5,003,178. This device includes a discharge cathode, a target anode, and a fine mesh grid spaced apart from the cathode a specified distance. Electrical bias of the grid allows control of the beam current. Scanning coils allow scanning the generated beam over a target.
To produce a more uniform beam, the grid may be arranged with varying depths or apertures. This arrangement can decrease the electrical field in the center of the discharge, decreasing the ion density at the center. This results in the attendant electron beam having a decreased electron density at the beam center, resulting in a more uniform beam. This is disclosed in U.S. Pat. No. 6,407,399, which teaches the use of a grid with apertures that are greater at the edges and smaller at the center.
If the ion beam is uniform, the electron beam uniformity depends only on the surface state of the cathode, which emits secondary electrons by ion bombardment. The secondary electron emission coefficient is a function of the material of which the cathode is comprised and the surface state of the cathode, which are highly dependent on the gasses absorbed by the cathode material.
Maintaining a clean cathode is critical to generation of a uniform and repeatable electron beam. However, since the target to which the electron beam is directed must frequently be introduced and removed from the vacuum chamber, there is an opportunity for contamination of the chamber with atmospheric gasses and impurities. These gasses and impurities may interact with the cathode surface, degrading the uniform emission from the cathode. To insure a uniform emission, the cathode is baked to clean surface impurities from the cathode. This is a time consuming and expensive process.
It is an object of the invention to provide a uniform wide area electron beam. It is a further object to utilize such an electron beam in a chamber for the treatment of target objects.